Context and Summary

Advanced LIGO

Gravitational waves offer a remarkable opportunity to see the universe from a new perspective, providing access to astrophysical insights that are available in no other way. The initial LIGO gravitational wave detectors have completed observations with their original design sensitivity, and the data have been interpreted to establish new upper limits on gravitational-wave flux. Another science run with modest improvements to the sensitivity is planned for the coming months.

The sensitivity of the initial LIGO instruments is such that it is perfectly possible that discoveries will be made. If they succeed, there will be a strong demand from the community to improve the sensitivity allowing more astrophysical information to be recovered from the signals. If no discovery is made, there will be no lesser urgency to improve the sensitivity of the instrument to the point where there is a general consensus that gravitational waves will be detected often and with a good signal-to-noise ratio. The development of the next generation of instrument must be pursued aggressively to make the transition from the initial to the Advanced detector in a timely way - after the complete science run of the initial detector, but as quickly as possible thereafter.

The Advanced LIGO detector upgrade meets these requirements for an instrument that will establish gravitational-wave astronomy. It is more than ten times more sensitive, and over a much broader frequency band, than initial LIGO. It can see a volume of space more than a thousand times greater than initial LIGO, and extends the range of compact masses that can be observed at a fixed signal strength by a factor of four or more.

This proposal to build Advanced LIGO has grown out of the LIGO Scientific Collaboration and has broad support both nationally and internationally from that community. A closely coordinated community R&D program, exploring the instrument science and building and testing prototype subsystem elements, has brought the design to a highly refined state. The LIGO Laboratory is leading and coordinating the fabrication and construction of the instruments, with the continued strong participation of the community.

The joint United Kingdom/German GEO Project has received funding to provide a capital investment in this construction project. The UK participants are applying these resources to providing the suspension subsystem, including suspension assemblies, their controls, and installation and commissioning. The German participants are undertaking the design and fabrication of the pre-stabilized laser subsystem. The GEO Project is a full partner in Advanced LIGO, participating at all levels in the effort.

Australian groups are also making capital contributions to Advanced LIGO. A consortium of Australian National University and the University of Adelaide are providing Hartmann phase sensors, a pre-lock length stabilization system, and specialized beam-pointing equipment to Advanced LIGO. ANU and Adelaide are full partners in Advanced LIGO.

Advanced LIGO can lead the gravitational-wave field to maturity.

The LIGO Mission

From its outset, LIGO has been approved by the National Science Foundation to directly observe gravitational waves from cosmic sources, and to open the field of gravitational wave astronomy. The program and mission of the LIGO Laboratory is to:

• observe gravitational wave sources,
• develop advanced detectors that approach and exploit the facility limits on interferometer performance,
• operate the LIGO facilities to support the national and international scientific community,
• provide data archiving for the LIGO data and contribute computational resources for the analysis of data,
• develop the software infrastructure for data analysis and participate in the search and analysis,
• and support scientific education and public outreach related to gravitational wave astronomy.

LIGO is envisioned as a new capability contained in a set of facilities and not as a single experiment. The LIGO construction project has provided the facilities that support the scientific instrumentation, and the initial set of laser interferometers to be used in the first LIGO scientific observation periods.

The facilities include the buildings and vacuum systems at the two observatory sites. The two observatories are located at Hanford, Washington and Livingston, Louisiana. The performance requirements on the LIGO facilities were intended to accommodate the initial interferometers and future interferometer upgrades and replacements, and possible additional interferometers with complementary capabilities. The requirements on the LIGO facilities were intended to permit future interferometers to reach levels of sensitivity approaching the ultimate limits of ground-based interferometers, limited by reasonable practical constraints on a large facility at a specific site.

Advanced LIGO represents the second generation of instruments to be installed in the LIGO infrastructure, and is expected to bring the science of gravitational radiation from a discovery mode to a mode of regular astrophysical observation.

    LIGO Detector Scientific Goals

The scientific program for LIGO is both to test relativistic gravitation and to open the field of gravitational wave astrophysics. More precise tests of General Relativity (and competing theories) will be made. LIGO will enable the establishment of a brand new field of astronomy, using a completely new information carrier: the gravitational field.

Initial LIGO represents an advance over all previous searches of two or three orders of magnitude in sensitivity and in bandwidth. Its reach is such that, for the first time, foreseeable signals due to neutron-star binary “inspirals” from the Virgo Cluster (15 Mpc distant) would be detectable. At this level of sensitivity, it is plausible, though not certain, that the first observations of gravitational waves will be made. If signals are not observed with initial LIGO, we will have set challenging upper limits on gravitational wave flux, far beyond the capability of any previously existing technology.

The Advanced LIGO interferometers proposed here promise an improvement over initial LIGO in the limiting sensitivity by more than a factor of 10 over the entire initial LIGO frequency band. It also increases the bandwidth of the instrument to lower frequencies (from ~40 Hz to ~10 Hz) and allows high-frequency operation due to its tunability. This translates into an enhanced physics reach that during its first several hours of operation will exceed the integrated observations of the 1 year LIGO Science Run. These improvements will enable the next generation of interferometers to study sources not accessible to initial LIGO, and to extract detailed astrophysical information. For example, the Advanced LIGO detectors will be able to see inspiraling binaries made up of two 1.4 M neutron stars to a distance of 300 Mpc, some 15x further than the initial LIGO, and giving an event rate some 3000x greater. Neutron star - black hole (BH) binaries will be visible to 650 Mpc; and coalescing BH+BH systems will be visible to cosmological distance, to z=0.4.

The existence of gravitational waves is a crucial prediction of the General Theory of Relativity, so far unverified by direct observation. Although the existence of gravitational radiation is not a unique property of General Relativity, that theory makes a number of unambiguous predictions about the character of gravitational radiation. These can be verified by observations with LIGO. These include probes of strong-field gravity associated with black holes, high-order post-Newtonian effects in inspiraling binaries, the spin character of the radiation field, and the wave propagation speed.

The gravitational wave "sky" is entirely unexplored. Since many prospective gravitational wave sources have no corresponding electromagnetic signature (e.g., black hole interactions), there are good reasons to believe that the gravitational-wave sky will be substantially different from the electromagnetic one. Mapping the gravitational-wave sky will provide an understanding of the universe in a way that electromagnetic observations cannot. As a new field of astrophysics it is quite likely that gravitational wave observations will uncover new classes of sources not anticipated in our current thinking.

Detector Design Fundamentals

The effect of a propagating gravitational wave is to deform space in a quadrupolar form. The effect alternately elongates space in one direction while compressing space in an orthogonal direction and vice versa, with the frequency of the gravitational wave. A Michelson interferometer operating between freely suspended masses is ideally suited to detect these antisymmetric distortions of space induced by the gravitational waves; the strains are converted into changes in light intensity and consequently to electrical signals via photodetectors.

Limitations to the sensitivity come from two sources: extraneous forces on the test masses, and a limited ability to sense the response of the masses to the gravitational wave strain. The thermally excited motion of the test mass and the suspension is a fundamental limitation, intrinsic to the way in which the measurement is performed; this influence is managed through the selection of low-mechanical-loss materials and designs which capitalize on them. Seismic motion causes forces on the mirrors due to the direct coupling through the isolation and suspension system, a technical noise source which is minimized through design; and due to the time-varying mass distribution near the mass (the Newtonian background).

Sensing limitations arise most fundamentally due to the statistical nature of the laser light used in the interferometry, and the momentum transferred to the test masses by the photons (linking the sensing and stochastic noise limitations to sensitivity). Technical noise sources that limit the ability to sense include frequency noise and intensity fluctuations in the laser light. Scattered light, which adds random phase fluctuations to the light, can also mask gravitational signals.

In the limit, valid for LIGO, that the instrument is short compared with the gravitational wavelength, longer arms give larger signals. In contrast, most competing noise sources remain constant with length; this motivates the 4km baseline of the Observatories. More generally, the scientific capability of LIGO is defined within the limits imposed by the physical settings of the interferometers and by the facility design, by the design of the initial detectors and ultimately by future interferometers designed to progressively exploit the facility capabilities.

Although the rates for gravitational wave sources have large uncertainty, an improvement in strain sensitivity linearly improves the distance searched for detectable sources. This increases the detection rate by the cube of the sensitivity improvement.

The Observatories

    LIGO Facility Scientific Capability

The LIGO facility design envisaged a progression of increasingly sensitive interferometers capable of extending the physics reach of the observatories. In the design of the observatories, LIGO incorporated critical design features into its facilities in order to optimize LIGO’s ultimate performance capabilities. These features include a building foundation and infrastructure which provides a clean, quiet environment for the instruments; a 4km long "L" ultra-high vacuum beam tube system that brings scattered light and index fluctuations due to residual gas to a negligible level; and a system of large vacuum chambers and pumping subsystems capable of providing a flexible envelope for a wide range of detector designs, and delivering a vacuum quality that complements the beam tube subsystem. Advanced LIGO requires no changes in this infrastructure to meet its scientific goals.

    The LIGO Observatories

LIGO Hanford Observatory (LHO), located on the U.S. Department of Energy Hanford site in eastern Washington, comprises 5 major experimental halls for the interferometer spread over 5 miles. 1.2-m diameter ultrahigh vacuum tubing connects these halls. Three support buildings house laboratories, offices, and an amphitheater, and two additional buildings are associated with maintenance and operations. Approximately 90,000 square feet of this space is under tight environmental control to minimize contamination of sensitive equipment. The physical plant has been designed to provide a low vibration environment similar to the surrounding undeveloped shrub-steppe environment.

LHO houses two interferometers; for initial LIGO, the instruments have arm lengths of 4 km and 2 km. For Advanced LIGO, both instrments will have 4km arm lengths. The beam tube can eventually accommodate up to 5 interferometer beams and the current station buildings can accommodate up to 3 interferometers to accommodate future growth.

Figure 1 LIGO Hanford Observatory (LHO) in aerial view. The 4-km interferometer arms are shown with the 5 main buildings along the orthogonal arm layout

Figure 2 LIGO Livingston Observatory (LLO) corner region in aerial view.

The LIGO Livingston Observatory, located in pine forests between Baton Rouge and New Orleans, Louisiana, is the site of a single 4-km laser interferometer gravitational wave detector. The beam tube dimensions are identical to those at LHO. The instrument design, and sensitivity, is the same as the 4km instrument at LHO, and multiple interferometers could be supported in the future.

Initial LIGO

The NSF Cooperative Agreement of May 1992 initiated LIGO Construction and Construction Related Research and Development. The Project schedule and cost estimates were reviewed by the NSF during September 1994 and presented to the National Science Board in November 1994. The LIGO construction effort was completed, on cost and close to schedule. 'First lock' of the initial LIGO instruments was acheived in 2000.

The instruments have shown a steady improvement in sensitivity, at all frequencies in the planned observation band, during the commissioning process. The sensitivity of the initial LIGO instruments is shown in Figure 3. All of the instruments met the sensitivity goal for initial LIGO of an RMS strain sensitivity of 10^-21 in a 100 Hz band in 2005. The present limits to performance are understood through a combination of measurement and system modeling.

Figure 3 This figure shows the progression in the strain sensitivity as a function of frequency. The Goal curve, and actual performance, exceeds the requirement by about a factor of three.

    Enhanced LIGO

Improvements to exceed the design sensitivity have been implemented and are now in commissioning. They involve some modest increases in laser power (using lasers contributed by our German GEO collaborators), and modifications to the electronics and optimization of control systems and filters. Many Advanced LIGO subsystems are used in prototype form in these enhancements to initial LIGO. Science runs with this new configuration will start in 2009.

    The LIGO Science Runs

The observatories have completed the definitive science run, S5 in the initial LIGO configuration. More than an integrated year of data was collected, taking into account duty cycle, maintenance, and tuning breaks. The LIGO Scientific Collaboration (LSC, please see below) analysis groups are undertaking the search for chirp signals from binary inspirals, periodic signals from neutron stars, burst signals from e.g., supernovae and gamma ray bursts, and from a possible stochastic noise background. The data analysis pipelines utilize a variety of sophisticated filtering techniques - templates, time-frequency analysis, inter-interferometer correlations, and use of the auxiliary and environmental data channels, as examples. Data are also being correlated with relevant optical data and, in the case of supernovae, with neutrino signals. Results from the analyses of this, and previous science runs, have been published in a range of scientific journals.

LIGO Scientific Collaboration

A fundamental goal of LIGO has been to become a true national facility available to the scientific community. In order to accomplish this, LIGO has broadened the participation to include the community of scientists interested in participating in the LIGO research program by creating the LIGO Scientific Collaboration (LSC). There are now some 650 members from 59 institutions in 11 countries. The LSC consists of both LIGO Laboratory scientists and those from collaborating groups. The LSC is organized so as to provide "equal scientific opportunity" to all members whether they are from within LIGO Laboratory or the broader LSC. It is growing steadily and will remain open to new members over the coming years. The international partners are involved in all aspects of the LIGO research program.

The full LSC collaboration meets twice yearly in an extended meeting, and various working groups meet more frequently. The LSC has produced White Papers that outline the plans for technical development of LIGO and for science data analysis. A publication policy and a conference committee are active, as well as the other functions necessary to make it a "full service" organization. the LSC works closely with the Virgo Collaboration, and the data from the LIGO detectors is combined with that of the Virgo detector (located near Pisa, Italy) with researchers from both collaborations sharing the analysis effort.

The Advanced LIGO design, both in basic conception and in the detailed R&D, is very much a product of the LSC (with a strong LIGO Laboratory element). The technical working groups have been and continue to be central to the advancement of the design, and this proposal is made with the strong support of the many participating institutions in the LSC.

LIGO has been organized such that the search for astrophysical signals and interpretations will be performed through the LSC. Preparation tasks for the runs are organized within the LSC, LSC members participate in the data taking runs, and the analysis of the data is coordinated through the LSC proposal driven process.

LIGO is available to all interested researchers through participation in the LSC, an open organization. To join, a research group defines a research program with the LIGO Laboratory through the creation of a Memorandum of Understanding (MOU) and relevant attachments. The group then presents its program to the LSC. When the group is accepted into the LSC it becomes a full scientific partner in LIGO.

Exploring the LIGO Capability: Advanced LIGO

As noted above, LIGO is designed to evolve and to support improvements in gravitational wave detectors. The initial LIGO infrastructure delivered its planned performance, and small advances in sensitivity at higher frequencies are underway. However, a large improvement requires an upgrade of the entire detector in a coordinated fashion. The considerable research and development in the Laboratory and the greater community that has taken place since the initial LIGO interferometer design was frozen enables this improvement. The Advanced LIGO instrument fulfills our requirements of a significant step forward in sensitivity, and can be delivered on a time scale that meshes with the end of the Initial LIGO observation plan.

The Advanced LIGO Project was thoroughly peer reviewed, approved by the National Science Board for funding, and received an official start under NSF MREFC support on 1 April 2008. Procurement and fabrication of interferometer elements is now underway.

    Overview of Advanced LIGO

The sensitivity goals for the Advanced LIGO detector systems are chosen to enable the advance from plausible detection to likely detection and rich observational studies of sources. These sensitivity goals require an instrument limited only by fundamental noise sources over a very wide frequency range. To achieve this sensitivity, almost every aspect of the interferometer must be revised from the initial LIGO design. The system briefly described below is the reference concept that is the basis for structuring the R&D program and the detailed studies of system tradeoffs performed as R&D results define the feasible parameters. A more complete description of the proposed detector, organized by subsystem, is found in the Advanced LIGO Reference Design. While still preliminary and subject to change, the curves for the strain sensitivity for various modes of operation can be found at Advanced LIGO anticipated sensitivity curves.

The basic optical configuration is a power-recycled and signal-recycled Michelson interferometer with Fabry-Perot "transducers" in the arms. Using the initial LIGO design as a point of departure, this requires the addition of a signal-recycling mirror at the output "dark" port, and changes in the interferometer readout and control systems. This additional mirror allows the gravitational wave induced sidebands to be stored or extracted (depending upon the state of "resonance" of the signal recycling cavity), and leads to a tailoring of the interferometer response according to the character of a source (or specific frequency in the case of a fixed-frequency source). The upgrade includes the three LIGO interferometers, allowing e.g., one interferometer at Hanford and the interferometer at Livingston to be tuned to be broadband, and the second interferometer at Hanford to be used as a higher-frequency narrowband detector.

To improve the quantum-limited sensitivity, the laser power is increased from the initial LIGO value of 10 W to ~200 W. The conditioning of the laser light follows initial LIGO closely, with a ring-cavity mode cleaner and reflective mode-matching telescope.

Whereas initial LIGO uses 25-cm, 11-kg, fused-silica test masses, the fused silica test mass optics for Advanced LIGO are larger in diameter (~34 cm) to reduce thermal noise contributions and more massive (~40 kg) to keep the radiation pressure noise to a level comparable to the suspension thermal noise. Compensation of the thermal lensing in the test mass optics (due to absorption in the substrate and coatings) is added to handle the much-increased power - of the order of 1 MW in the arm cavities.

The test mass is suspended by fused silica fibers, in contrast to the steel wire sling suspensions used in initial LIGO. The resulting suspension thermal noise is anticipated to be less than the radiation pressure noise (in broad-band observation mode) and to be comparable to the Newtonian background ("gravity gradient" noise) at 10 Hz. The complete suspension has four pendulum stages, contributing to the seismic isolation and providing multiple points for actuation.

The seismic isolation system is built on the initial LIGO piers and support tubes but otherwise is a complete replacement, required to bring the seismic cutoff frequency from 40 Hz (for initial LIGO) to 10 Hz. RMS motions (frequencies less than 10 Hz) are reduced by active servo techniques. The result is to render the seismic noise negligible at all observing frequencies. Through the combination of the seismic isolation and suspension systems, the required control forces on the test masses will be reduced by many orders of magnitude in comparison with initial LIGO, reducing also the probability of non-Gaussian noise in the test mass.

The overall performance of Advanced LIGO is dominated at most frequencies by the quantum noise of sensing the position of the test masses, with a contribution at mid-frequencies from the internal thermal noise of the test masses. This design, with modest enhancements after it enters scientific use, should take this interferometer architecture to its technical endpoint; it is as sensitive as one can make an interferometer based on familiar technology: a Fabry-Perot Michelson configuration with external optical readout using room temperature transmissive optics. Further advances will come from R&D that is just beginning, such as the exploration of cryogenic optics and suspensions, purely reflective optics, and a change in the readout to one which fully exploits our understanding of the quantum nature of the measurement (e.g., quantum non-demolition speed meters). These later developments will be timely for instruments to be developed in the second decade of this century.

    Advanced LIGO R&D

The LIGO Laboratory detector R&D has been directed at the challenges posed by the Advanced LIGO design for the last decade. This R&D program is a significant part of the larger R&D program of the LIGO Scientific Collaboration. The LSC program has been developed in a collaborative manner and is coordinated through the LSC Working Groups and by the LIGO Laboratory.

The Advanced LIGO R&D program is largely complete, and the Advanced LIGO construction phase is ramping up. Formally, the R&D phase for a given subsystem ends when the subsystem is ready for fabrication, and the Project takes over the management of the effort and resources.

The activities carried out prior to construction funding included small-scale fundamental research motivated by Advanced LIGO. Examples of this are studies of core optic substrate absorption, measurement of mechanical losses in suspension fibers, and studies of candidate photodetectors for the gravitational wave readout. The R&D program also includes large-scale prototypes of subsystems such as full-scale seismic isolation systems, full-scale suspensions, and full-scale core optics. In order to carry out this research, in most cases these components have been fully engineered to the realistic requirements and configuration of Advanced LIGO. In order to study the performance and control of a suspension subsystem, for example, the prototype studied must represent the Advanced LIGO design down to details such as suspension fiber material, bonding technique, as well as control electronics design, component selection, and physical layout. This kind of rigorous full-scale development program is needed to reduce risks prior to defining and committing to a construction project and to minimize the time between installation and the start of observations at the design sensitivity.

For more detailed information on the Advanced LIGO techical design please see the Advanced LIGO Reference Design document.

Advanced LIGO Schedule

The Advanced LIGO fabrication and construction schedule grows out of the tightly coordinated R&D program currently concluding. The objectives in establishing the schedule were to

• Allow the initial LIGO instruments to be fully exploited, and in particular to ensure the commitment to a full integrated year of observation with initial LIGO instruments.
• Allow a complete R&D cycle, with extensive testing of final designs, before committing to fabrication.
• Bring the Advanced LIGO instruments on-line as quickly as possible to meet the demands of the community for the observing capability of the Advanced LIGO detector.

The schedule is based on our extensive experience with the design, fabrication, construction, and commissioning of the Initial LIGO detectors.

The highlights of the schedule are

• Receipt of funding for the fabrication and construction project in 2008.
• Delivery of first interferometer hardware to the observatory staging facilities in 2009.
• Decommissioning of initial LIGO at the LIGO Livingston Observatory in 2011, and simultaneous start of installation of Advanced LIGO there.
• Decommissioning of initial LIGO at the LIGO Hanford Observatory in 2012, and simultaneous start of installation of Advanced LIGO there.
• Both observatories in commissioning by 2013.
• Both observatories in operation by 2014.